The present invention relates generally to semiconductor processing, and more particularly to an apparatus and method for keeping a floating wafer centered within the apparatus.
Dutch Patent 1003538 describes the individual treatment of wafers. A wafer is accommodated in a reactor, with the distance to the walls of the reactor being set to be comparatively small. It is thus possible for a particularly rapid hear transfer to take place. It is possible to heat the wafer to well above 1000xc2x0 C. within a few seconds. Since the wafer in principle does not have to be supported during this heating operation, but rather is held accurately in position by the gas streams, local temperature differences caused by mechanical contact are avoided. The very uniform heating of the wafer allows said short heating times to be achieved without the wafer becoming warped. Due to the high capacity of the reactor and the relatively low heat capacity of the wafer, in combination with the efficient heat transfer between the walls of the reactor and the wafer, so that the wafer quickly reaches the temperature of the reactor wall, this rapid heating of the wafer can be achieved with a relatively low peak capacity of the heating means in the reactor. The rapid heating of the wafer enables the total treatment time to be reduced, with the result that such a treatment is able to compete with the batchwise treatment of a series of wafers, while the consequences of problems remain limited to only one wafer.
In the prior art, it is proposed to provide the bottom part of the chamber with pins which can be displaced in the vertical direction in order to move the wafer into the loading and unloading position. During operation, such pins are inactive, since the wafer is then to be in the floating position. By suitably controlling the supply of gas for the various openings which lie on opposite sides of the wafer within the limits of the chamber in which the wafer is accommodated, the latter can be held floating in a very stable manner.
Although in the floating position the wafer is absolutely stable in the axial (vertical) direction, it has been found that the positioning in the radial (horizontal) direction provides problems: the wafer is able to move too easily in the radial direction. This is because of the position of the gas-supply and gas-discharge openings in the reactor, which openings can be regarded as an air bearing or gas bearing. In a radial air bearing which is known in the prior art, the air is supplied at the outside of the disc which is to be supported, and the air is discharged via an opening in the center of the disc which is to be supported; this leads to stable radial positioning. However, this results in considerable pressure differences across the surface of the wafer, while a requirements for a uniform process result is that the range of pressure variation across the wafer is no greater than xc2x12.5%. In the reactor described in Dutch Patent 1003538, stable positioning is achieved by positioning the gas-discharge openings along the circumferential side of the reactor wall.
Therefore, the object of the present invention is to provide a device which allows extremely accurate radial positioning of the wafer during operation, i.e. during treatment.
This object is achieved in accordance with one aspect of the invention, with a device for accommodation of a wafer. The device has an accommodation chamber formed by an upper part and a bottom part. The lower part has openings that extend between the accommodation chamber and an underside of the lower part. The openings provide for supply of treatment gas into the accommodation chamber in order to place the wafer in a floating position. Equipment in proximity of an end section of the accommodation chamber is configured to stabilize the wafer in a predetermined floating position. The equipment has a gas discharge and a gas supply that open into the accommodation chamber. The gas discharge and the gas supply are configured so that, upon a displacement of the wafer from the predetermined floating position, a flow resistance between the gas discharge and the gas supply increases at a side of the wafer that faces a direction of displacement such that an increased pressure urges the wafer back in the predetermined floating position.
According to a further possible embodiment, the gas is fed near the circumference edge of the wafer. At approaching the feed opening by the edge of the wafer locally the pressure increases and the wafer is moved back to the intended position. It will be understood that also combinations are possible of feed and discharge openings to obtain the centring effect described above.
According to a preferred embodiment of the invention, an additional opening is provided for introducing of a positioning-gas near the extremity of the wafer i.e. near the discharge for the gases. This gas moves over a part of the surface of the wafer which is not used to any beneficial effect and does not create any significant pressure differences. As a result, this gas can be supplied in relatively large quantities without producing unacceptable pressure differences, i.e. supplying gas for positioning purposes does not effect the treatment of the wafer, but does stabilise its radial position in the chamber in question. According to one proposal, such pressure differences are limited to xc2x12.5%.
Basically, there are two possible solutions for the positioning of the gas-discharge opening and the positioning-gas supply opening respectively.
According to a first embodiment, the center axis of the gas-discharge line is coaxial with the center axis of the central plane of the treatment chamber. In this case, it is assumed that the wafer is situated in the center of the treatment chamber in the position of use. Movement of the wafer towards the gas-discharge opening results in this opening being closed off to a greater or lesser extent, thus producing a force which pushes it back. In this design, the positioning-gas supply openings are preferably arranged substantially perpendicular to the center plane of the wafer to be treated.
According to an embodiment, these positions of gas-supply and gas-discharge openings are precisely reversed.
Both the gas-discharge opening and the positioning-gas supply opening may be designed in any way which is known in the prior art. The positioning-gas supply opening may comprise a number of openings which are arranged along (part of) a circle. If a complete circle is described, the position of the wafer is controlled in all radial directions. It is also possible for only part of a circle to be described, which is important if the wafer comes out of a supply channel which is connected to the treatment chamber. The positioning-gas supply opening may also be designed as a continuous slot or as a succession of elongate openings. The same applies to the discharge opening, which may be designed as a continuous ring or as a large number of openings or a number of slots. All this depends on the intended design and the desired operating conditions.
Since the positioning-gas is in principle not involved in the treatment of the wafer, it is possible to select a much less expensive gas, such as nitrogen, for this positioning-gas. It is, however, desirable for the gas to be supplied at the same temperature as the actual process gas, in order in this way to eliminate the effects caused by temperature differences as far as possible.